As a method to increase transmission capacity in an optical transmission system, multilevel-coded signal transmission is well known in the art (see Shigeki Hayase, et al., “8-State per Symbol, 30-Gbit/s Optical Modulation/Demodulation Scheme,” The 2003 Institute of Electronics, Information and Communication Engineers, Communication Society Conference). A standard multilevel modulating method in a conventional multilevel-coded signal transmission system uses optical-phase modulation such as PSK (Phase Shift keying) and DPSK (Differential Phase Shift keying), and optical-intensity modulation together. For instance, the above reference discloses a 3-bit per symbol modulating method combining QPSK (Quadrature Phase Shift Keying) and binary ASK (Amplitude Shift keying) together.
However, QPSK requires complicated precoding before modulation. In the case that DQPSK (Differential Quadrature Phase Shift Keying) is used, it requires complex precoder and decoder.
On the other hand, when optical intensity modulation and optical phase modulation are used together, there is a problem that optical phase modulation cannot be applied to a bit slot with zero optical intensity. Furthermore, in optical-intensity modulation, it is difficult to accomplish multilevel of 3-bit per symbol or more.